36 Scientific Proceedings, Royal Dublin Society. 



the seventh study a small vessel was mentioned, which takes its 

 origin from the anterior side of the hyoid artery. Dohrn now 

 considers this artery as the original ventral portion of the spiracular 

 artery. (See fig. vn.) 



The question now arises, what were the functional motives 

 which led to the change of condition which we perceive at present ? 

 The answer to this must be sought in the formation of the mouth. 

 The relative enlargement of the oral aperture in elasmobranchs 

 must have constituted the most important cause in forcing back 

 the gills, lying posteriorly to the mouth. A fusion of gills and 

 vessels was the result of this pressure. Probably the ventral 

 portion of the posterior spiracular vein and the anterior hyoid vein 

 became fused. On that account a direct communication of the 

 posterior hyoid vein with the spiracular artery resulted, and after 

 the ventral part of the posterior spiracular vein and the anterior 

 hyoid vein had disappeared, the passage of the blood from the 

 posterior hyoid vein to the spiracular artery remained by means of 

 the hyoid commissure. (See fig. vn.) 



The connexion of vessels of the teleostean pseudobranch and that 

 of the selachian spiracle with the conus arteriosus are identical. 

 Dohrn concludes, therefore, that the pseudobranch of teleosteans is 

 homologous with the selachian spiracle. The efferent vessels of 

 the teleostean pseudobranch, likewise, arise exactly as in elasmo- 

 branchs. The main vessel opens directly into the posterior carotid, 

 a branch is connected with the ganglion oculomotorii, just as in 

 elasmobranchs, and then runs on to the posterior side of the bulb. 

 Gradually, networks of vessels are produced, which increase in 

 number, forming the peculiar body known as choroidal gland. 

 Later on, the vessel going to this gland becomes the more 

 powerful, while the main vessel to the posterior carotid grows 

 smaller, and at last it loses all connexion with it. 



The pseudobranchia of teleosteans is formed from the anterior 

 wall of the original spiracular cleft, and is separated from the 

 hyomandibular by the remaining lumen of the spiracular pouch. 

 If the spiracular cleft had not disappeared, says Dohrn, no one 

 would have thought of taking the pseudobranchia for anything but 

 the spiracular gill of teleosteans. 



Dohrn concludes the last study but one, by making a few 

 remarks on the opercular gill of ganoids, which he studied from 



